Abstract

Aims Syntaxin (Syn)-1A binds sulfonylurea receptor (SUR) nucleotide binding folds of cardiac myocyte (SUR2A) and islet β-cells (SUR1) to inhibit ATP-sensitive potassium (K_ATP) channels. We further reported that Syn-1A reduced the potency and efficacy of β-cell-specific K_ATP channel openers (KCOs). Here, we examined whether Syn-1A would influence non-specific (diazoxide) and SUR2-specific KCOs [N-cyano-N′-(1,1-dimethylpropyl)-N″-3-pyridylguanidine (P-1075) and cromakalim] on cardiac myocyte K_ATP channels activation.

Methods and results Confocal microscopy and Western blotting verified the presence of both Syn-1A and -1B expressions on rodent cardiac ventricular myocytes. Inside-out patch–clamp electrophysiology was utilized to examine the effects of these syntaxins on K_ATP macroscopic currents activated by various KCOs from a stable cell line expressing the potassium inward rectifier 6.2 (Kir6.2)/SUR2A and from C57BL/6 male mouse ventricular myocytes. Syn-1A inhibited the current amplitude activated by P-1075, cromakalim and diazoxide via its H3 but not Habc domain. Syn-1B exhibited similar inhibitory effects on P-1075 activation of K_ATP currents. In examining for direct effects of Syn-1A on the KCO binding to cardiac SUR2 receptors, we found that Syn-1A did not directly affect [³H]-P-1075 binding to rat cardiac membrane SUR2A at maximum binding capacity, but was able to mildly reduce the affinity of cold P-1075 and cromakalim to displace [³H]-P-1075 binding.

Conclusion In conclusion, Syn-1A (and Syn-1B) could inhibit K_ATP currents activated by SUR2A-acting KCOs. Potential fluctuations in the levels of these syntaxins in the myocardium may affect the therapeutic effectiveness of cardiac KCOs.